The present invention relates to a brake device for a vehicle such as a large vehicle and a construction machinery vehicle provided with a Diesel engine, a torque converter with a lockup clutch and a power shift transmission.
In a vehicle such as a large vehicle and a construction machinery vehicle having a large gross vehicle weight, a kinetic energy during downhill running or high speed running is very large, and a braking efficiency to be effected by a service brake device only becomes insufficient. In general, such a defect is compensated by providing an auxiliary brake device such as an engine exhaust brake.
FIG. 5 shows a conventional auxiliary brake device. Referring to FIG. 5, during normal running of the vehicle, power of a Diesel engine 1 is transmitted through a torque converter 2, transmission 4 and axle 5 to tires 6, and the vehicle travels at a speed corresponding to a gear range of the transmission 4 selected by a shift lever.
During running of the vehicle as mentioned above, an engine speed N.sub.E of the engine 1 is detected by an engine speed sensor 13, and a vehicle speed V is detected by a vehicle speed sensor 14. A turbine runner speed N.sub.T of the torque converter 2 is computed according to the vehicle speed V. As shown in FIG. 7, when the turbine runner speed N.sub.T is lower than a predetermined speed a, the lockup clutch 3 is disengaged to generate slip in the torque converter 2. Thereafter, when the turbine runner speed N.sub.T becomes the predetermined speed a or more, the lockup clutch 3 is engaged to eliminate the slip in the torque converter 2 and directly transmit the power between the engine 1 and the transmission 4.
When the vehicle is braked by the engine exhaust brake during running, a magnetic valve 10 is operated by an exhaust brake switch 11 and an accelerator switch 12 interlocking with an accelerator pedal to supply a compressed air to an exhaust brake cylinder 9 and operate the cylinder 9. As a result, an exhaust brake closing valve 8 provided in an exhaust pipe 7 of the Diesel engine 1 is operated by the cylinder 9 to thereby brake the engine 1.
Upon application of the engine exhaust brake, the control of the exhaust brake is effected by the operation of the switches 11 and 12 irrespective of a gear range of the transmission 4 and an on/off state of the lockup clutch 3 in the torque converter 2.
FIG. 6 shows control characteristics among an engine speed, engine braking force and engine exhaust braking force in relation to a vehicle speed in the case that the transmission 4 is a three-speed transmission. Referring to FIG. 6, the engine speed (shown by chain lines Ia, IIa, IIIa), the engine braking force (shown by broken lines Ib, IIb, IIIb) and the engine exhaust braking force (shown by solid lines Ic, IIc, IIIc) are controlled according to each gear range.
When the turbine runner speed N.sub.T of the torque converter 2 becomes the predetermined value a or more, the lockup clutch 3 is engaged to eliminate the slip in the torque converter 2, thereby effectively applying the engine brake and the engine exhaust brake. However, when the vehicle speed V increases excessively during downhill running, for example, the power is transmitted from the tires 6 to the engine 1 because the lockup clutch 3 is engaged. As a result, the engine speed N.sub.E of the engine 1 exceeds a permissible maximum speed N.sub.max, that is, the engine 1 overruns to cause damage of the engine 1.
Accordingly, when the vehicle speed V is excessively increased during downhill running, for example, the operator must select a low gear range having a large reduction ratio of the transmission 4, that is, manually down-shift the gear range under the effectiveness of the engine exhaust brake, so as to reduce the vehicle speed. In this manner, such down-shift operation is troublesome. Further, if the operator fails to down-shift the gear range, the vehicle runs with a high gear range at high speeds in the on-state of the lockup clutch 3. Accordingly, a large braking force of the auxiliary brake device such as engine brake and engine exhaust brake cannot be obtained, and resultantly the service brake device is excessively heated or damaged. Further, when the operator down-shifts the gear range in the on-state of the lockup clutch 3 as mentioned above, a large reduction ratio is suddenly provided in association with the effectiveness of the engine exhaust brake to cause rapid deceleration and braking. As a result, an effective range of the vehicle speed is narrowed to remarkably reduce a running efficiency. Additionally, during normal running on a horizontal road, a high speed gear range having a small reduction ratio of the transmission 4 is usually selected, and the down-shift is not carried out every time the engine exhaust brake is applied. Accordingly, the overrun of the engine 1 tends to be generated.
To prevent the overrun of the engine 1, another auxiliary brake device such as a hydraulic brake device (hydraulic retarder) 15 and an electric brake (eddy current retarder) 16 as shown by an alternate long and two short dashes line in FIG. 5 may be provided in addition to the engine exhaust brake. However, the additional auxiliary brake device causes complication in structure of the brake system and also causes an increase in vehicle weight and cost.